1. Field of the Invention
The present invention relates to the processing of integrated circuit wafers and equipment useful for such processing. More particularly, the invention relates to processes and equipment for indexing of a cassette in a vacuum environment. The invention uses a motion actuator fully disposed within the vacuum enclosure to eliminate the large vacuum to atmosphere interface found in prior art devices.
2. Background of the Related Art
In the processing of semiconductor wafers, the processes are often carried out in a vacuum to reduce the particulate level to which the wafers are exposed. The introduction of the wafers into the vacuum processing system must be made at atmospheric pressure. The system must then be pumped down to the desired vacuum. Therefore, each time wafers are introduced into the system, the system must go through a pumpdown cycle creating a throughput "hit," or delay, during which time the system must sit idle. Each throughput hit reduces the efficiency of the system and the manufacturing process.
Due to the importance of throughput efficiency, as many wafers as possible are introduced into the system during each pumpdown cycle. This is accomplished by placing the wafers in a cassette that holds the wafers in a horizontally oriented, vertically offset arrangement. The cassette is then placed on a receiving tray in a loadlock which is subsequently sealed and pumped down to the desired vacuum.
Vacuum processing systems, however, typically operate in a single horizontal plane. Robots within the system move the wafers horizontally through the system and position the wafers in a series of processing stations in which a series of process steps are carried out.
Consequently, the wafers vertically disposed within the cassette must be sequentially indexed to the horizontal operating plane so that the robot can access the indexed wafer and move it through the system. Accordingly, the loadlock must contain an indexing apparatus to adjust the vertical position of the cassette and align each of the wafers within the cassette with the horizontal operating plane.
In typical prior art devices, an example of which is shown in FIG. 1, the indexing apparatus uses a support shaft attached to the bottom of the receiving tray supporting the wafer cassette. The support shaft extends through the bottom of the vacuum enclosure thereby requiring a seal to enable the enclosure to be pumped to a desired vacuum level. A motion actuator positioned external to the loadlock moves the support shaft vertically in response to a control signal and, thereby, indexes the wafer cassette. The prior art motion actuator used for indexing wafer cassettes use either a lead screw or hydraulic offset mechanism.
In these prior art devices, a collapsible bellows extends between the receiving tray and the bottom of the loadlock to provide a vacuum to atmosphere seal where the support shaft penetrates the loadlock wall. The collapsible bellows expands and contracts as the receiving tray moves up and down to maintain the seal. However, this vacuum to atmosphere interface is a source for leaks which result in system inefficiency and downtime.
Additionally, the lead screw or hydraulic offset mechanisms of the prior art are relatively imprecise which lessens the cassette placement accuracy and the repeatability of the mechanism. Also, these lead screw or hydraulic offset mechanisms extend outside the walls of the loadlock and, therefore, require additional space, a precious commodity in manufacturing clean rooms.